1. Field of the Invention
The present invention relates to material analysis, and more particularly to material analysis using thermography.
2. Discussion of Prior Art
In known lock-in thermography material analysis a sample of a material to be analysed is positioned between a halogen lamp and an infra-red camera. The lamp is switched on and off by a sinusoidally modulated signal so that heat pulses are emitted towards the sample at a particular frequency, generating sinusoidal thermal waves inside the material. Reflections produced by defects and interfaces (normally boundaries between different materials) in the sample interfere with incoming waves transmitted by the lamp to produce a wave pattern at the surface of the material. The infra-red camera captures a thermal image of the wave pattern at the surface of the material.
U.S. Pat. No. 5,711,603, D. J. Roth et al., Res. Nondestr. Eval. 9 (1997) 147–169, EP0089760 A2 and GB2235604 A all disclose transient thermography techniques for nondestructive testing of material samples.
As the thermal images produced by the camera are captured at the same moment as when the halogen lamp is switched on, the system does not allow for thermal lag. Our studies have shown that it takes an appreciable amount of time for the heat pulses produced by the lamp to be transmitted from the surface of the sample facing the lamp through the thickness of the sample to the sample surface facing the camera. As a result of this, the camera is unable to accurately locate the sinusoidally varying heat pulses and so a poor signal-to-noise ratio is produced. Accordingly, the thermal images produced by the camera are of a “grainy” nature and therefore less useful for analysing imperfections in the material sample. It is often desirable to study imperfections at different depths within the sample. The limitations of the prior art lock-in thermography apparatus described above mean that it does not allow sufficient discrimination of imperfections at selected depths in the sample.
An aim of the present invention is to provide a system which produces improved thermal images for material analysis. One embodiment of the invention is intended to allow thermal images to be produced which represent selected points within the thickness of the material sample.